Background Traumatic brain injury (TBI) can cause long-lasting sequelae that may increase the risk of suicidal or criminal behaviour, but large-scale longitudinal studies are lacking on the link between TBI and events of suicide attempt and violent crime. This study examined the incidence of suicide attempt and violent crime following hospital contact for TBI in a nationwide cohort study. Methods We used nationwide register data covering all individuals aged 10+ living in Denmark during 1980-2016 (n = 7 783 951). Of these, 587 522 individuals had a hospital contact for TBI. Incidence rate ratios (IRR) were calculated by Poisson regression analyses while adjusted for relevant covariates including other fractures and psychiatric diagnoses. Results Individuals with TBI had higher rates of suicide attempt (females IRR, 2.78; 95% CI 2.71-2.85; males IRR, 3.00; 95% CI 2.93-3.08) compared to individuals without TBI in adjusted analyses. Multiple TBI and temporal proximity to TBI were associated with higher rates of suicide attempt. Individuals with TBI had higher rates of violent crime (females IRR, 2.43; 95% CI 2.36-2.49; males IRR, 1.80, 95% CI 1.78-1.82) compared with individuals without TBI. Higher rates of violent crime were found after multiple TBI and temporal proximity to TBI. Conclusions This nationwide cohort study found higher rates of suicide attempt and violent crime among individuals with prior hospital diagnosed TBI, compared with individuals without TBI. This emphasises the need for preventive efforts immediately after TBI diagnosis, which might mitigate the risks of a trajectory toward suicidal or violent behaviours.